1. Field of the Invention
This invention relates to an intake system for an internal combustion engine, and more particularly to an intake system for an internal combustion engine in which the volumetric efficiency is improved by the ram effect (inertia effect) of intake air.
2. Description of the Prior Art
As is well known, a negative pressure wave generated in an intake system of an internal combustion engine upon the initiation of each intake stroke is propagated upstream of the intake system and is then reflected at an end of the system opening to the atmosphere or to a surge tank disposed on an upstream side of the intake system toward the intake port as a positive pressure wave. By arranging the intake system so that the positive pressure wave reaches the intake port immediately before closure of the intake valve to force intake air into the combustion chamber, the volumetric efficiency can be improved.
However, in order to sufficiently improve the volumetric efficiency, the intake passage must be long when the engine speed is relatively low and must be short when the engine speed is relatively high.
Thus, there has been proposed an intake system in which the length of the intake passage is changed according to the engine speed in order to obtain an inertia effect of intake air over a wider engine speed range. For example, in the intake system disclosed in Japanese Unexamined Patent Publication No. 56(1981)-115819, each of the discrete intake passage portions leading to the respective combustion chambers is bifurcated to form a long passage portion and a short passage portion both opening to a surge tank or the like at the upstream end, and an on-off valve is provided in the short passage portion to open the short passage portion in a high engine speed range to shorten the effective length of the discrete intake passage portion, thereby obtaining a sufficient inertia effect of intake air in the high engine speed range in addition to in a low engine speed range.
In the intake system described above, the volumetric efficiency for one cylinder is improved by the inertia effect of intake air generated by pressure propagation only in the discrete intake passage portion leading to the cylinder. If the pressure propagation in the discrete intake passages leading to other cylinders can be effectively utilized, the volumetric efficiency will be able to be improved by the inertia effect of intake air over a wider engine speed range. This can be realized by providing a communicating passage interconnecting the short passage portions. However, how to provide the communicating passage in the limited space around the engine is a difficult problem. The long passage portions must be long enough to obtain the inertia effect in the low engine speed range and must be smooth in shape so that intake air can smoothly flow therethrough. On the other hand, the short passage portions must be short enough to obtain the inertia effect in the high engine speed range. These requirements further add to the difficulties in providing the communicating passage.
Further, preferably the communicating passage is small in volume in view of pressure transfer between the cylinders. Thus, there is involved another difficulty in providing the communicating passage which is limited in volume and can interconnect the short passage portions, satisfying the requirements described above.